body{
    margin: 0;
    padding: 0;
    background: #f1f1f1;
}
#menu{
    /* border: 1px solid #333; */
    margin-top: 340px;
}
#menu li{
    /* li前面的点消失 */
    list-style: none;
    display: inline-block;
}
#menu li a{
    /* 下划线消失 */
    text-decoration: none;  
    position: relative;
    color: #313131;
    font-size: 50px;
    font-weight: 700;
    font-family: sans-serif;
    display: block;
    overflow: hidden;
    transition: 0.7s all;
    padding: 14px 30px;
    text-transform: uppercase;
}
#menu li a::before{
    content: '';
    width: 60px;
    position: absolute;
    bottom: 0;
    right: 100%;
    border-bottom: 8px solid #333;
    transition: 0.5s all;
}
#menu li a:hover::before{
    right: 0;
}